Frequency modulated OFDM over various communication media

ABSTRACT

The present invention provides an FM Orthogonal Frequency Division Multiplexing (OFDM) modulation process that enable high-speed data communications over any transmission media and networks. The process is implemented with a modem device modulator and demodulator that provides communication with several other modem devices along any communication media that uses an FM OFDM modulation technique, a physical transmission medium such as power lines, or wireless (air), or cable, or twisted pairs communication media.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of pending U.S. patent applicationSer. No. 10/770,342, filed Feb. 2, 2004, which is incorporated byreference herein in its entirety.

BACKGROUND

The present invention relates generally to telecommunication devices,and more particularly to a modulation technique used for enabling andtransmitting various media types and digital data at very high speedsover any communication media.

Typically, a telecommunication modem is composed of two components. Thefirst component is the modulator, which typically modulates digital dataand provides that modulated data to the transmitter. The secondcomponent is the demodulator that demodulates the modulated data back todigital data.

OFDM (Orthogonal Frequency Division Multiplexing) or FDM (FrequencyDivision Multiplexing) is used in the telecommunication industry.Although it can be combined with adaptive coding and modulationtechniques, such as flexible selection among different codes and codingrate, 64 QAM, 32 QAM, 16 QAM, QPSK and BPSK constellations, to approachchannel capacity, certain Signal to Noise Ratio (SNR) is required foreach coding and modulation combination to achieve a desired Bit ErrorRate (BER) performance. Even higher SNRs are needed if the system adoptslarger alphabet size for higher data rate. Also, adaptive coding can beused with OFDM based on SNR variations. For example, when the signal isheavily attenuated through the channel, the same data can be repeatedseveral times to boost the SNR at the receiver end for a reliablerecovery. However, this would reduce the throughput of the communicationsystem.

In order to achieve desired SNRs for reliable communication and keep thethroughput high, the present invention intends to Frequency Modulate(FM) the OFDM data stream before transmission and FM demodulate the RFdata stream before demodulating the OFDM data stream.

OFDM can gain 10-20 dB SNR that will result in significantly longerdistance communication without increasing the transmit power level andsignificantly higher throughput to keep the high speed for thecommunication media. These communication media include but are notlimited to AC and DC power line carrier communication, wirelesscommunication, cable, telephone lines, twisted pairs, and coaxial cablecommunications. DC power line communications includes the communicationover the DC wiring harness for moving vehicles like trucks, buses, SUV'sand etc.

SUMMARY OF THE INVENTION

Briefly stated, in a first embodiment, the present invention defines anew modulation technique that enables high-speed data communicationsover any transmission media and networks. The FM OFDM modulationtechnique for communication comprises:

-   -   a modem device that provides communication with several other        modem devices along any communication media that uses FM OFDM        modulation technique,    -   a physical transmission medium, including but not limited to        various power lines, or wireless (air), or cable, or twisted        pairs communication media.    -   a modem device modulator that receives bit streams from video,        voice, internet or other digital data sources, and which        consists of a FM OFDM modulator that comprises:        -   1. receiver feedback information retrieval        -   2. coding scheme selection        -   3. coding and interleaving        -   4. constellation selection among BPSK, QPSK, 16 QAM, etc.        -   5. digital modulation        -   6. pilot symbol insertion        -   7. carrier mapping        -   8. IFFT (Inverse Fast Fourier Transfer)        -   9. adding cyclic prefix        -   10. peak limiting        -   11. low pass filtering        -   12. ADC (analog to digital converter)        -   13. FM (Frequency Modulator) for I and Q    -   and a modem device demodulator that sends bit streams to video,        voice, internet or other data sources, and which consists of a        FM OFDM demodulator that comprises:        -   1. decoding and de-interleaving        -   2. demodulation (BPSK, QPSK, 16 QAM, etc.)        -   3. channel estimation        -   4. carrier demapping        -   5. FFT (Fast Fourier Transfer)        -   6. cyclic prefix removal        -   7. frequency offset estimation and compensation        -   8. synchronization and timing offset estimation        -   9. DAC (digital to analog converter)        -   10. FM (Frequency Demodulator)

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe preferred embodiments of the invention, will be better understoodwhen read in conjunctions with the appended drawings. For the purpose ofillustrating the invention, there are shown in the drawings embodimentsthat are presently preferred. It should be understood, however, that theinvention is not limited to the precise arrangement andinstrumentalities shown. In the drawings, like numerals are used toindicate like elements throughout. In the drawings:

FIG. 1 is a graphical illustration of the OFDM (Orthogonal FrequencyDivision Multiplexing) transmitter (modulator) for power line carrier(PLC) communication that includes a power line coupler, RF and analogcircuitry, in accordance with one preferred embodiment of the presentinvention.

FIG. 2 is a graphical illustration of the OFDM (Orthogonal FrequencyDivision Multiplexing) receiver (demodulator) for power line carriercommunication that includes a power line coupler, RF and analogcircuitry, in accordance with one preferred embodiment of the presentinvention.

FIG. 3 is one of the graphical illustrations of the FM (FrequencyModulated) OFDM (Orthogonal Frequency Division Multiplexing) transmitter(modulator) for power line communication that includes a power linecoupler, RF and analog circuitry, in accordance with one preferredembodiment of the present invention.

FIG. 3A is another graphical illustration of the FM (FrequencyModulated) OFDM (Orthogonal Frequency Division Multiplexing) transmitter(modulator) for power line communication that includes a power linecoupler, RF and analog circuitry, in accordance with one preferredembodiment of the present invention.

FIG. 4 is one of the graphical illustrations of the FM (FrequencyDemodulated) OFDM (Orthogonal Frequency Division Multiplexing) receiver(demodulator) for power line communication that includes a power linecoupler, RF and analog circuitry, in accordance with one preferredembodiment of the present invention.

FIG. 4A is another graphical illustration of the FM (FrequencyDemodulated) OFDM (Orthogonal Frequency Division Multiplexing) receiver(demodulator) for power line communication that includes a power linecoupler, RF and analog circuitry, in accordance with one preferredembodiment of the present invention.

FIG. 5 is one of the graphical illustrations of the FM modulator usingDirect Digital Synthesizer (DDS), in accordance with one preferredembodiment of the present invention.

FIG. 6 is another graphical illustration of the FM modulator, inaccordance with one preferred embodiment of the present invention.

FIG. 7 is a graphical illustration of a typical FM demodulator, inaccordance with one preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention presents improvements to an OFDM or FDM basedcommunication application by FM modulating the OFDM or FDM signalsbefore transmission and FM demodulating the carrier frequencies beforeOFDM or FDM demodulation.

FM modulation is well known to be used for a number of applicationswhere long distance communications are important. Typically, FMmodulation can gain 10-40 dB SNR (signal to noise ratio) depending onthe modulation factor that is being used. Bandwidth determines the SNR.Larger bandwidth communication devices have smaller SNR than smallerbandwidth devices. To reach high speed communication, the requirementswill be higher bandwidth. If single or a couple of carrier frequenciesare being used for communication, then the bandwidth will need to belarger and therefore the SNR will be lower. Even if FM modulation isused for high speed data communication, the SNR will be lower than anOFDM system.

For example, if one needs to reach 10 Mbps speed over a communicationmedia by using a 250 Mhz FM modulated carrier frequency over a 60 Mhzbandwidth, then the level of noise will be about −67 dBm. Evenrecovering about 15-20 dB SNR by using FM still will not providereliable long distance communications because the noise level is toohigh. By using 1024 carriers for OFDM with 30 Mhz bandwidth at a centerfrequency of 250 Mhz, one can reach up to 150 Mbps speed with a noiselevel of −86 dBm at every 30 Khz carrier frequency. Consequently OFDMcould reach longer distance and higher speed communication than FMalthough it will depend on the transmitted power too. OFDM will be abetter choice for FCC emission consideration.

Block-Wise Description of the OFDM or FDM System from Modulator'sViewpoint

-   -   1. feedback from the receiver is retrieved to make a judgment on        the forward direction channel quality;    -   2. the code selection unit picks up a coding scheme based on the        previously made channel quality judgment, different codes can be        used for higher data rate at the target performance requirement,        for example a rate ½ code can be used to replace a rate ⅓ code        when the SNR is high;    -   3. in the coding and interleaving unit, digital streams are        encoded with the selected code, either a convolutional or a        block code, the output of the encoder is interleaved so that        errors due to channel distortion scatter across the stream        independently;    -   4. the constellation selection unit picks up a signal        constellation, or alphabet, among BPSK, QPSK, QAM etc. based on        the previously made channel quality judgment, different        alphabets can be used for higher data rate at the target        performance requirement, for example 64-QAM or 16-QAM can be        used to replace QPSK or BPSK when the SNR is high;    -   5. the modulator maps binary output from the coding and        interleaving unit to real or complex numbers in the chosen        alphabet by the constellation selection unit;    -   6. pilot symbols are inserted into the symbol stream from the        modulator based on a channel estimation algorithm of different        kinds;    -   7. output of the previous unit is then mapped onto specific bins        of an Inverse Fast Fourier Transform (IFFT) unit;    -   8. IFFT unite conducts discrete Fourier transform on the input        data;    -   9. the tailing part of an IFFT output block is prepended to the        block in this unit to combat the distortion due to multipath        propagation, the prepended part is called Cyclic Prefix (CP);    -   10. a limitor is used to make sure the signal is within the        amplifier's dynamic range;    -   11. an optional digital low pass filter is used to confine the        signal within the desired band, this functionality can also be        achieved with an analog low pass filter.    -   12. the in-phase (I) and quadrature (Q) signal components are        then fed to two separate Digital to Analog Converters (DACs).        The analog outputs of the DACs are amplified, filtered and mixed        with the local oscillator (LO) signal.    -   13. the mixer output is then sequentially filtered, amplified        and fed to a coupler or antenna.    -   14. the coupler or antenna injects carrier frequency signals        into physical media.        Block-wise Description of the OFDM or FDM System from        Demodulator's Viewpoint

In the analog part, the carrier frequency signals retrieved fromphysical transmission media by means of a coupler is amplified, filteredand mixed to generate the intermediate frequency (IF) signal, the IFsignal is further amplified, filtered and mixed to generate the basebandsignal. This baseband signal is again amplified and filtered. The I andQ components of the filter output are fed into two separate Analog toDigital Converters (ADCs). The digitized I/Q components then propagatethrough the following blocks:

-   -   1. The digital I/Q signals first go through a unit that finds        the start and end of a data frame;    -   2. The second unit estimates and compensates for frequency        offset, i.e., the difference between transmit LO frequency and        receive LO frequency embedded in the I/Q samples;    -   3. The third unit removes the cyclic prefix of each IFFT block;    -   4. The fourth unit conducts Fast Fourier Transform (FFT) on each        IFFT block after CP is removed;    -   5. Carrier demapper separates the pilot and information bearing        bins in the output of FFT unit;    -   6. The channel estimation unit generates estimates of noise        power and channel gain at each FFT bin;    -   7. With the help of channel gain and noise power estimates, the        demodulator generates hard or soft decisions on the modulating        bits;    -   8. The de-interleaver permutes the hard or soft decisions and        then feeds its output to a decoder which generates bit streams        to be delivered to the upper layer applications such as        internet, video/audio/telephone/data.

FIG. 1 shows the transmitter, coupler and Orthogonal Frequency DivisionMultiplexing (OFDM) Modulator, which is a multi-carrier modulationscheme by means of Fast Fourier Transform (FFT). This modulation schemetakes as input bit streams 1 from any application, such asvoice/video/audio/internet. It retrieves forward channel qualityfeedback from the receiver 42 and selects a good code 40, such as ablock code or a convolutional code, to encode the bit streamsaccordingly, then interleaves 2 the encoded bits to break error burstsat the receiver end into scattered individual errors across the wholetransmission. The Receiver feedback retrieval 42, the Code Selection 40,the Constellation selection 41, coding & interleaving 2, and modulation3 accomplish the adaptive coding and adaptive modulation for the OFDMsystem. The OFDM system further converts interleaved bits into real orcomplex symbols in an alphabet 3 which are selected 41 based on channelcondition feedback. Different alphabets can be used for a higher datarate at the target performance requirement, for example 64-QAM or 16-QAMcan be used to replace QPSK or BPSK when the channel signal to noiseratio is high. After pilot symbols 4 are inserted, the composite symbolstream modulates specific carriers 5 via Inverse Fast Fourier Transform(IFFT) 6. A tailing part is copied to the beginning of the IFFT resultsto make the Cyclic Prefix (CP) 7. A limitor 8 is used to make sure thedigital signal is within the amplifier's dynamic range. An optionaldigital low pass filter 9 can also be used to confine the signal withinthe desired band. This functionality may alternatively be achieved withan analog low pass filter. The in-phase (I) and quadrature (Q)components of the digital signal are now fed to two separate Digital toAnalog Converters (DACs) 10. The analog outputs of the DACs areamplified, filtered and mixed 11 with the local oscillator (LO) signal.The mixer output is sequentially filtered, amplified and sent to acoupler or antenna 12, which will transmit the DSP carrier frequencysignals to the communication media 25 like power line, air, coax cable,or twisted pair cable.

FIG. 2 shows the receiver, coupler and Orthogonal Frequency DivisionMultiplexing (OFDM) Demodulator. In the analog part, the carrierfrequency signal retrieved from physical transmission media 25 by meansof a coupler or antenna 12 is amplified, filtered and mixed 11 togenerate the intermediate frequency (IF) signal, and the IF signal isagain amplified, filtered and mixed 11 to generate the base band signal.This base band signal is further amplified and filtered. The I and Qcomponents of the filter output are sent to two separate Analog toDigital Converters (ADCs) 13. From the digitized I/Q components, theframe header is located by means of synchronization algorithms 14.Frequency offset 15 or the difference between transmit LO and receive LOis estimated and compensated. Based on the knowledge of the frameheader, the cyclic prefix 16 of each transmitted IFFT block is removedand Fast Fourier Transform (FFT) 17 is conducted. From obtained resultson pilot-bearing carriers 18, channel estimation algorithm 19 deliversnoise power and channel gain estimates which facilitates the generationof hard or soft decisions on the encoded bits 20. The hard or softdecisions are finally de-interleaved and fed to the decoder 21 thatgives bit streams 22 transmitted for upper layer applications such asinternet, video/audio/data.

FIG. 3 shows the Frequency Modulated (FM) Orthogonal Frequency DivisionMultiplexing (OFDM) Modulator which is a multi-carrier modulation schemeby means of Fast Fourier Transform (FFT). This modulation scheme takesas input bit streams 1 from any application, such asvoice/video/audio/internet. It retrieves forward channel qualityfeedback from the receiver 42 and selects a good code 40, such as ablock code or a convolutional code, to encode the bit streamsaccordingly, then interleaves 2 the encoded bits to break error burstsat the receiver end into scattered individual errors across the wholetransmission. The Receiver feedback retrieval 42, the Code Selection 40,the Constellation selection 41, coding & interleaving 2, and modulation3 accomplish the adaptive coding and adaptive modulation for the OFDMsystem. The OFDM system further converts interleaved bits into real orcomplex symbols in an alphabet 3 which are selected 41 based on channelcondition feedback. Different alphabets can be used for a higher datarate at the target performance requirement, for example 64-QAM or 16-QAMcan be used to replace QPSK or BPSK when the channel signal to noiseratio is high. After pilot symbols 4 are inserted, the composite symbolstream modulates specific carriers 5 via Inverse Fast Fourier Transform(IFFT) 6. A tailing part is copied to the beginning of the IFFT resultsto make the Cyclic Prefix (CP) 7. A limitor 8 is used to make sure thedigital signal is within the amplifier's dynamic range. An optionaldigital low pass filter 9 can also be used to confine the signal withinthe desired band. This functionality may alternatively be achieved withan analog low pass filter. The in-phase (I) and quadrature (Q)components of the digital signal are now fed to two separate Digital toAnalog Converters (DACs) 10. The analog outputs of the DACs areamplified, filtered, I and Q is converted together 23 and FM Modulated24 with the local oscillator (LO) signal. The FM Modulator 24 output issequentially filtered, amplified and sent to a coupler or antenna 12,which will transmit the DSP carrier frequency signals to thecommunication media 25 like power line, air, coax cable, or twisted paircable.

FIG. 3A shows another embodiment of the Frequency Modulated (FM)Orthogonal Frequency Division Multiplexing (OFDM) Modulator, when I andQ base band converter 23 is not developed yet, which is a multi-carriermodulation scheme by means of Fast Fourier Transform (FFT). Thismodulation scheme takes as input bit streams 1 from any application,such as voice/video/audio/internet. It retrieves forward channel qualityfeedback from the receiver 42 and selects a good code 40, such as ablock code or a convolutional code, to encode the bit streamsaccordingly, then interleaves 2 the encoded bits to break error burstsat the receiver end into scattered individual errors across the wholetransmission. The Receiver feedback retrieval 42, the Code Selection 40,the Constellation selection 41, coding & interleaving 2, and modulation3 accomplishes the adaptive coding and adaptive modulation for the OFDMsystem. The OFDM system further converts interleaved bits into real orcomplex symbols in an alphabet 3 which are selected 41 based on channelcondition feedback. Different alphabets can be used for a higher datarate at the target performance requirement, for example 64-QAM or 16-QAMcan be used to replace QPSK or BPSK when the channel signal to noiseratio is high. After pilot symbols 4 are inserted, the composite symbolstream modulates specific carriers 5 via Inverse Fast Fourier Transform(IFFT) 6. A tailing part is copied to the beginning of the IFFT resultsto make the Cyclic Prefix (CP) 7. A limitor 8 is used to make sure thedigital signal is within the amplifier's dynamic range. An optionaldigital low pass filter 9 can also be used to confine the signal withinthe desired band. This functionality may alternatively be achieved withan analog low pass filter. The in-phase (I) and quadrature (Q)components of the digital signal are now fed to two separate Digital toAnalog Converters (DACs) 10. The analog outputs of the DACs areamplified, filtered, and I and Q is separately FM Modulated 24 with twodifferent local oscillator (LO) signals. The two FM Modulators 24 outputwill create 2 separate frequency bands F1 and F2 (for I and Q) forcommunication which is sequentially filtered, amplified and sent to acoupler F1 and coupler F2 or antenna F1 and antenna F2 12, which willtransmit the DSP carrier frequency signals to the communication media 25like power line, air, coax cable, or twisted pair cable.

FIG. 4 shows the receiver, coupler, FM demodulator and OrthogonalFrequency Division Multiplexing (OFDM) Demodulator. In the analog part,the carrier frequency signal retrieved from physical transmission media25 by means of a coupler or antenna 12 is amplified, filtered and mixed11 to generate the intermediate frequency (IF) signal, and the IF signalis again amplified, filtered and FM Demodulated 26 to generate the baseband signal. This base band signal is further converted 27 into I and Q,amplified and filtered. The I and Q components of the filter output aresent to two separate Analog to Digital Converters (ADCs) 13. From thedigitized I/Q components, the frame header is located by means ofsynchronization algorithms 14. Frequency offset 15 or the differencebetween transmit LO and receive LO is estimated and compensated. Basedon the knowledge of the frame header, the cyclic prefix 16 of eachtransmitted IFFT block is removed and Fast Fourier Transform (FFT) 17 isconducted. From obtained results on pilot-bearing carriers 18, channelestimation algorithm 19 delivers noise power and channel gain estimateswhich facilitates the generation of hard or soft decisions on theencoded bits 20. The hard or soft decisions are finally de-interleavedand fed to the decoder 21 that gives bit streams 22 transmitted forupper layer applications such as internet, video/audio/data.

FIG. 4A is another embodiment of the FM OFDM demodulator when the baseband converted 27 into I and Q is not developed yet. In this case, I andQ is separately FM demodulated from two different frequency bands likeF3 and F4 and sent to the OFDM demodulator for data recovery. In theanalog part, the two carrier frequency band signal F3 and F4 isretrieved from physical transmission media 25 by means of a coupler F3and F4 or antenna F3 and F4 12 is amplified, filtered and mixed 11 togenerate the intermediate frequency (IF) signal, and the IF signal isagain amplified, filtered and F3 and F4 frequency bands are separatelyFM Demodulated 26 to generate the base band signal. These base band Iand Q signals separately are further amplified and filtered. The I and Qcomponents of the filter output are sent to two separate Analog toDigital Converters (ADCs) 13. From the digitized I/Q components, theframe header is located by means of synchronization algorithms 14.Frequency offset 15 or the difference between transmit LO and receive LOis estimated and compensated. Based on the knowledge of the frameheader, the cyclic prefix 16 of each transmitted IFFT block is removedand Fast Fourier Transform (FFT) 17 is conducted. From obtained resultson pilot-bearing carriers 18, channel estimation algorithm 19 deliversnoise power and channel gain estimates which facilitates the generationof hard or soft decisions on the encoded bits 20. The hard or softdecisions are finally de-interleaved and fed to the decoder 21 thatgives bit streams 22 transmitted for upper layer applications such asinternet, video/audio/data.

FIG. 5 shows an embodiment of an FM modulator which uses AD9852Integrated Circuits (IC) from Analog Devices, Inc. This IC is a DirectDigital Synthesizer (DDS) 30 and includes the D/A converters 10 and theFM modulator 24.

FIG. 6 shows another embodiment of an FM modulator 64 that uses a PhaseLock Loop (PLL) IC 75 such as the ADF4111 IC, a Voltage controlledoscillator (VCO) 74 such as the MC12148 IC (available from Motorola),and a varactor diode 76 such as the MA4ST083CK (available from M/A-COM,Lowell, Mass.). The varactor 76 oscillates with the L46 inductor tocreate the oscillation frequency and the FM modulation, while the PLL 75controls and stabilizes the carrier frequency for the VCO 74.

FIG. 7 shows an embodiment of the FM demodulator 26 where an MC13155D(available from Motorola) wideband FM limiter IC is used with a RLC tankcircuit 84 that determines the carrier frequency and its necessarybandwidth for detection.

Another example of an FM demodulation scheme is to use a PLL with VCOand charge pump.

The challenge to do FM OFDM is that OFDM carrier frequencies do not comeout as a flat frequency response from the D/A converters. The magnitudevariations between each carrier frequency can usually reach 12 dB. Thus,the OFDM processing needs to include more processing power and highersampling rates in order to get the OFDM carrier frequencies smallermagnitude variations.

For power line communication, any type of coupler can be used. Thepresent invention prefers to use a capacitive coupler orresin/dielectric-filled coupler for overhead or for underground powerlines. A coupler is a transformer. Thus, a transformer can be acapacitive transformer to become a capacitive coupler. A resin-filledcoupler means that a transformer is filled in resin. A dielectric-filledcoupler means that a transformer is filled with a dielectric material. Acapacitive transformer can also be filled with resin or other dielectricmaterial. The transformer primary impedance is the same order ofmagnitude as that of overhead or underground power lines or larger. Acapacitive coupler with a data port may be used here to couple a modemto the line. The line may be terminated with a resistance approximatelyequal to the characteristic impedance of the power transmission cable.Modem means herein is the modulator and demodulator. The secondary sideof a capacitive coupler that connects to the transmitter and/or receiveris wound about equal number of turns as the primary side of the couplerthat is connected through a capacitor to the power line. The coupler hasa core between the primary and secondary winding. This coupler can beused for any high voltage power line between 120V AC to 750 KV AC withthe necessary safety changes. The most preferred coupler for power linecommunication is the matching coupler that can match the power linecharacteristic impedance to the coupler and the coupler can also matchthe characteristic impedance of the transmitter receiver at anytime andany location.

Changes can be made to the embodiments described above without departingfrom the broad inventive concept thereof. The present invention is thusnot limited to the particular embodiments disclosed, but is intended tocover modifications within the spirit and scope of the presentinvention.

What is claimed is:
 1. An apparatus constructed to transmit acommunication signal over a transmission medium, said apparatuscomprising: digital signal processing units constructed to process anincoming baseband signal, wherein the baseband signal is a digitallymodulated with a digital modulation technique selected based on aretrieved transmission medium quality feedback and outputting either afrequency division multiplex (FDM) output signal or an orthogonalfrequency division multiplex (OFDM) output signal; characterized in thatthe apparatus further comprises a first frequency modulation (FM)modulator, said first FM modulator receiving said frequency divisionmultiplex (FDM) output signal or the orthogonal frequency divisionmultiplex (OFDM) output signal as the output signal of said digitalsignal processing units and outputting a frequency modulated signal tothe transmission medium wherein the frequency modulated signal gains asignal to noise ratio being by 10 dB to 40 dB larger than the signal tonoise ratio of the frequency division multiplex (FDM) output signal orthe orthogonal frequency division multiplex (OFDM) output signal suchthat the frequency modulated output signal is suitable for long distancecommunications.
 2. The apparatus of claim 1, further including a secondFM modulator, wherein the output signal of the digital signal processingunits consists of an in-phase signal and a quadrature signal, thein-phase signal being provided to the first FM modulator and thequadrature signal being provided to the second FM modulator, the firstand the second FM modulators each outputting a frequency modulatedoutput signal to the transmission medium.
 3. An apparatus constructed toreceive a communication signal from a transmission medium, saidapparatus comprising: a plurality of digital signal processing unitsbeing constructed to process either an orthogonal frequency divisionmultiplex (OFDM) communication signal or a frequency divisionmultiplexing (FDM) communication signal, wherein the communicationsignal is digitally demodulated with a digital modulation techniqueselected based on a retrieved transmission medium quality feedback, andoutputting a baseband signal; characterized in that the apparatusfurther comprises a first frequency modulation (FM) demodulator, saiddigital signal processing units being operatively connected to an outputof the first frequency demodulator, said first frequency demodulatorbeing constructed to receive an output from the transmission medium andoutputting a frequency demodulated frequency division multiplex (FDM)signal or an orthogonal frequency division multiplex (OFDM) signal;wherein a frequency modulated signal for long distance communicationsreceived as an output from the transmission medium gains a signal tonoise ratio being by 10 dB to 40 dB larger than the signal to noiseratio of the frequency demodulated frequency division multiplex (FDM)output signal or the frequency demodulated orthogonal frequency divisionmultiplex (OFDM) output signal outputted from the first frequencydemodulator.
 4. The apparatus of claim 3, wherein an output of thetransmission medium consists of an in-phase signal and a quadraturesignal, the in-phase signal being provided to the first frequencydemodulator and the quadrature signal being provided to a secondfrequency demodulator, the first and the second FM demodulators eachoutputting a frequency demodulated FDM signal to the digital signalprocessing units.